This invention relates to diesel engine exhaust particulate filters of honeycomb structure, hereinafter referred to as "DPF's", and, in particular, to improved DPF's for smaller displacement diesel engines.
It is well known that solid particulates and larger particles may be filtered from fluids by passing the particulate contaminated fluids through porous walled honeycomb structures. U.S. Pat. No. 4,276,071 and a pair of pending applications Ser. Nos. 165,646 and 165,391, both filed July 3 , 1980 and assigned to the assignee hereof, the latter now U.S. Pat. No. 4,329,162 describe and claim honeycomb filters for removing carbonaceous solid particulates from diesel engine exhausts and other filtering applications. Each DPF has a multiplicity of interconnected thin porous walls which define at least one inlet surface and one outlet surface on the filter and a multiplicity of hollow passages or cells extending through the filter from an inlet surface or an outlet surface or both. Inlet cells are formed open at at least one inlet surface to admit contaminated fluid into the filter. The inlet cells are closed where they adjoin any outlet surface of the filter. Outlet cells are formed open at an outlet surface to discharge fluid which has passed through the filter. The outlet cells are similarly closed where they adjoin any inlet surface. The interconnected thin walls are provided with an internal interconnected open porosity which allows the fluid to pass from the inlet to the outlet cells while restraining a desired portion of the solid particulates in the fluid from passing therethrough. The particulates are trapped in or collected on the surfaces of the thin walls defining the inlet cells. As the mass of collected particulates increases, back pressure across the filter increases and/or the flow rate of fluid through the filter decreases until an undesirable level of back pressure and/or flow rate is reached and the filter either is regenerated by removal of the trapped particulates or discarded. DPF's are typically installed in a housing which is inserted like a muffler or catalytic converter into the exhaust system of a diesel engine equipped vehicle.
Existing U.S. Government regulations (Title 40 Consolidated Federal Regulations .sctn..sctn.86.082-8,86.082-9, July 1, 1981 Rev.) of the Environmental Protection Agency ("EPA") limit solid particulate emissions from light duty diesel engine equipped vehicles to 0.6 grams per mile ("gm./mi.") in 1982. A 0.2 gm./mile limit has been mandated for 1985 vehicles. Maximum emissions of larger displacement (i.e., 8 cylinder and larger 6 cylinder) diesel engines have been on the order of about 0.4 to 0.7 gm./mile in a normal driving cycle. For example, in the aforesaid application Ser. No. 165,646, the 1980 Oldsmobile 350 CID (5.9 liter) V-8 engine used for comparatively testing filter compositions and configurations generated particulates at an average rate, per 4 cylinder bank of the engine, of between about 0.17 and 0.24 gm./mi. under steady state moderate speed operation (approximately 30 mph constant). The same engine would produce particulates at average rates of about 0.5 and 0.6 gm./mi. during a normal driving cycle. The term "normal driving cycle" is used in the same manner as it is used in E.P.A. regulation to refer to the test(s) under which a light duty vehicle diesel engine is certified for use in the United States. See Title 40, Consolidated Federal Regulations 86.101 et. seq. (July 1, 1981, Rev.). Lower emission rates, about 0.35 gm./mi. or less, are typically generated by much smaller (i.e. about 3.0 liter or less total cylinder displacement) six and four cylinder diesel engines which are being offered in 1981 or have been proposed for future models. These include a variety of diesel engines being manufactured by various Japanese concerns ranging from a 2.8 liter, 6 cylinder engine by Nissan down to a 2.0 liter, 4 cylinder engine manufactured by Toyo Kogyo as well as the 1.6 liter Volkswagon diesel engine. Recent certification testing of diesel engines for use in 1982 light duty vehicle engines, including larger V-6 and V-8 engines to be offered, will generate particulates at average rates of about 0.45 gm./mi. or less over normal driving cycles and a significant majority of those engines will generate particulates at average rates of about 0.35 gm./mi. or less or the normal driving cycle. Thus, DPF's or other apparatus will not be required in most diesel engine equipped vehicles for 1982 but apparently will be required in virtually all vehicles equipped with existing types of diesel engines by 1985 to meet the federal diesel particulate emission restrictions.
Certain considerations are helpful in evaluating and comparing DPF's. One is filter efficiency, that is, the amount of suspended particulates which are removed from the fluid as it is passed through the filter (hereinafter expressed as a percentage of the weight of all particulates in the fluid prior to its passage through the filter). Desirably, no more efficiency is provided than is absolutely necessary as increased efficiencies typically increase filter back pressure and reduce useful operating time. Another consideration is pressure drop across the filter, that is the difference between the fluid upstream and downstream from the filter caused by the presence of the filter and particulates therein. As pressure drop increases so does the engine exhaust back pressure. Useful operating time is the cumulative time of service of a DPF until its pressure drop and engine exhaust back pressure become sufficiently great as to necessitate the regeneration or replacement of the filter. Another significant consideration is compact structure, the smallest space-saving volume and configuration of the filter for obtaining the best combination of filter efficiency, back pressure characteristics and useful operating time. Other desirable features include thermal shock resistance, mechanical strength and non-deleterious reaction with the exhaust gases.
The U.S. Pat. No. 4,276,071 describes the characteristics of a DPF having a single inlet end face and a single outlet end face on opposing surfaces of the filter and a multiplicity of cells extending through the filter in a mutually parallel fashion between the inlet and outlet end faces. This type of filter is hereinafter referred to as a "unidirectional flow-type" DPF. The thin walls forming the cells of the filter are described as having an open porosity with a 10% internal open volume formed by pore spaces having mean sizes ranging between about 2 to 15 microns. The patent further describes using a pair of such filters with an 8 cylinder, V-type automotive diesel engine, each filter being connected with one of the engine's two banks of four cylinders.
The applications Ser. Nos. 165,646 and 165,391 describe several ceramic batch mixtures for fabricating DPF's with porous thin walls having internal open volumes ranging between about 35% and 66% formed by pores having mean diameters ranging between about 4 and 35 microns. The application Ser. No. 165,391 particularly claims a field of DPF thin wall open porosities having open pore volumes and mean pore sizes ranging between about 35 to 90% and about 1 to 20 microns, respectively. It is believed that filters formed with thin walls having open porosities lying within the field to have acceptable pressure drop characteristics and filtration efficiencies of about 75% or more in desired filter configurations, levels which would be needed to assure meeting the 1985 federal particulate emission standards with the largest displacement diesel engines being offered in the 1980-1981 period, and to provide extended useful operating life when compared with the performance of a comparably dimensioned DPF having the thin wall open porosity disclosed in the U.S. Pat. No. 4,276,071.
It would be desirable to identify a field of thin wall open porosities to meet the proposed 1985 light duty vehicle diesel engine particulate emission requirements (i.e. 0.2 gm./mi.) as was done in the aforesaid Ser. No. 165,391 application and optimized for use with the existing design (i.e. 1981) smaller displacement diesel engines as well as with the majority of improved diesel engines (i.e. 1982) of all displacements so as to provide greater filter operating life or more compact filter size or both.